Minggu, 11 Oktober 2009

Heat of Formation Worked Example Problem

1. Heat of Formation Problem

Calculate ΔH for the following reaction:

8 Al(s) + 3 Fe3O4(s) --> 4 Al2O3(s) + 9 Fe(s)

Heat of Formation Solution

ΔH for a reaction is equal to the sum of the heats of formation of the product compounds minus the sum of the heats of formation of the reactant compounds:

ΔH = Σ ΔHf products - Σ ΔHf reactants

Omitting terms for the elements, the equation becomes:

ΔH = 4 ΔHf Al2O3(s) - 3 ΔHf Fe3O4(s)

The values for ΔHf may be found in the Heats of Formation of Compounds table. Plugging in these numbers:

ΔH = 4(-1669.8 kJ) - 3(-1120.9 kJ)

ΔH = -3316.5 kJ

Answer

ΔH = -3316.5 kJ

2. Heat of Formation Problem

Calculate ΔH for the ionization of hydrogen bromide:

HBr(g) --> H+(aq) + Br-(aq)

Heat of Formation Solution

ΔH for a reaction is equal to the sum of the heats of formation of the product compounds minus the sum of the heats of formation of the reactant compounds:

ΔH = Σ ΔHf products - Σ ΔHf reactants

Remember, the heat of formation of H+ is zero. The equation becomes:

ΔH = ΔHf Br-(aq) - ΔHf HBr(g)

The values for ΔHf may be found in the Heats of Formation of Compounds of Ions table. Plugging in these numbers:

ΔH = -120.9 kJ - (-36.2 kJ)

ΔH = -84.7 kJ

Answer

ΔH = -84.7 kJ

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